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<br />Frequency <br /> <br />to CrOSS flooded areas. Ingeneral,floodwaterthreeormorefeet <br />deep and flowlng at e velocity of three or more feet per second <br />oouldeasily sweep an adult..erson off his feet, thuscreatlngdefl- <br />nlte danger of inJury or drowning. Rapidly rlslngMd swiftly flowing <br />floocwater may trap persons in homes that may be destroyed, or in <br />vtihlcles that are ultimately submerged or floated. WaTer lines can <br />be ruptured by deposits of debris and the force of floodwater thus <br />creating the possibIlity of contaminated domestIc water supplies. <br />Sanitary Stlwer'!, bacOtntl pressure lintls,biowlng"""nholtlcoverstospe.. <br />raw sewage intothefloodwaters,andstockyardwast"eaddstothe <br />pollution and hazard of epidemIc. <br />Water flowing in excess of four fe..t per second Is cap- <br />able of transporting sedIment and c::luslng severe erosion of stream <br />banks and embankments around bridge abutmflnts. The ~"tLJratl<">n ..tf"ct~ <br />of long.dur",tion flooding c<ln weaken bridge abutments, levees and <br />other embankments so they tall as the floodwaters recede. <br /> <br />Stand1'lrOProJectFloo<I <br />The $tan{~ard Project Flood Is del i ned as a major flood <br />that can beexpeded to occur tror-1'l severe combination 01 rneteor- <br />ological and hydrological conditions that Is considered reasonably <br />charaderistlcofthe geographlcare1'l In which the study area is lo- <br />cated, excl~dlng extremeiy rare combl~ations. The Corps o~ Engineers, <br />in cooperation with NOAA ~'eather Servl ce, has made comprehens i ve <br />studies and irtvestlg<ltions based on tll& past reoords of experienced <br />storms and floods and has developed generalized procedures for esti- <br />matIng the flood potential of storms. Severe as the maximum known <br />flood may have been on any given stream, It Is generally accepted <br />that a larger flood will sooner or later occur. The magnItude of <br />the Standard Project and Interrnod;ateReglonal Floods are shown in <br />Table 3. <br /> <br />The IntennedlateRegiortal Flood has, by defInition, an <br />average occurrence frequency of once In 100 years, or a 1% ch1lnce <br />01 bel ng <x:ualod or exceeded in any one ye~r period. I tis i rnprac~ <br />tlca I, hOwever, to ass 19" a frequency to t~e greater Standdrd Pro- <br />jectl"lood. For purposes of hazard evaluation, It is important to <br />recogniz& that either 01 tha5a major floodS Can occur In ~ny year. <br />Floods larger than tile Standard Project Flood are possIble, but the <br />COmtlinationoffactorsnecessarytoproducesuch largetlowwould <br />rarely occur. <br /> <br />~ou I d create conei tlo"~ "aZ,lr~ous -:-0 p,,'.s,,;.ns ar,c vec, :.c :es at-:-"mp7i ng <br /> <br />Flooded Areas and FlOOd Demaqa <br />,~reas -:-hat .. i II be f loodoc by the I nterClQdl "te Regional <br />ilnd Stand::lN Project FloodS are Indicated On the flocced area maps <br />of Plate 2 tllrcugh Plate II. Reference 11MS en the"", plates indi- <br />cate the elevation of tho IntermedIate Regional Flood aMd ara shown <br />at five foot Intervals. Gy sub-:-ractl~g the ground elevations <br />from the approprlat& reference elevation tho flood dopth ca~ ~e d~- <br />termlnedforanypoinT. These elevation reference lines also repre- <br />sent the minlMu"n olevatlon for flood-proofing. FlOOd profiles On <br />Plates 12 through PI ate 21 I ndicate the Alavatioils of both f loorJ~ <br />il~C Can be us&d for Getermi n i ng flood depth5 or f loodproof I ng e ieva- <br />rloos. Typlca I c~oss sections on Plate 22 through P I ate 24 are 9 <br />represantO'ti ve secti o~s from a~proxi mately /0 sections Obtai nOd from <br />f lei d survel's "nd photograph Ie fOOa~s. The location 01 all CrOSS <br />5{K,tlons are shc~'n em th{' profile ~y e heevy Ii,., 'md "U"l~(lr. <br />Floocwater depths d?Te~i~ed for any rartitular ~ross ~ection are <br />'lot alwJYs i nd'{;Jtl ve 01 t.'1& de~fhs ,ltta In'ed. FlOOdwaters .,.l){;uen't Iy <br /> <br />HazarC5 of Large Floods <br />The extent and dd'",ge caused by .3ny flood dApends on <br />th8 topogrepny of the area flooded, d&pth and duratlOl1of flooding, <br />velocity of flo.., r~te of rise, and developments in the flood plaln. <br />An I ntar1'"l!ldl ate Regional Flood on Cottonwood Cre'ek would result in <br />the i nundJtion ot resi dentl~ 1 1InC corrmorcl<ll properties in the study <br />areil. ~ep tlood~ater tlowing at high v,,;ocitv dnd carrying cJebris <br /> <br />I') <br /> <br />iF> <br />